AttributeError: 'list' object has no attribute '...'

Question

Hello, I created a subscriber node to get data from a topic. Topic broadcasts humans on the camera in a list called human_list. I want to read this data with below code. However when I rosrun it I get an error stating this

[ERROR] [1522830692.692015]: bad callback: <function callback at 0x7f7c0598aaa0>
Traceback (most recent call last):
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/rospy/topics.py", line 750, in _invoke_callback cb(msg)
  File "/home/robolab3/catkin_ws/src/hand_raise_detector/scripts/armraiselistener.py", line 8, in callback
    x = human.body_key_points_with_prob.x
AttributeError: 'list' object has no attribute 'x'

rosmsg show openpose_ros_msgs/OpenPoseHumanList is as follows:

msgs/OpenPoseHumanList
  std_msgs/Header header
    uint32 seq
    time stamp
  string frame_id
  std_msgs/Header rgb_image_header
    uint32 seq
    time stamp
    string frame_id 
  int32 num_humans
  openpose_ros_msgs/OpenPoseHuman[] human_list
    int32 num_body_key_points_with_non_zero_prob
    openpose_ros_msgs/PointWithProb[18] body_key_points_with_prob
      float64 x
      float64 y
      float64 prob

My code to subscribe this topic is as follows. I appreciate your help to identify what about my code is wrong.

import rospy
from openpose_ros_msgs.msg import OpenPoseHumanList
def callback(msg):
  for human in msg.human_list:
      x = human.body_key_points_with_prob.x
      y = human.body_key_points_with_prob.y
      prob = human.body_key_points_with_prob.prob
      rospy.loginfo('x: {}, y: {}, prob: {}'.format(x, y, prob))
def armraiselistener():
 rospy.init_node('armraiselistener', anonymous=True)
 rospy.Subscriber("/openpose_ros/human_list", OpenPoseHumanList, callback)
 rospy.spin()
if __name__== '__main__':
 armraiselistener()

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Originally posted by tolga-uni-lu on ROS Answers with karma: 33 on 2018-04-04

Post score: 0

Answer 1

def callback(msg):

  for human in msg.human_list:
      x = human.body_key_points_with_prob.x
      ...

This is a Python issue more than a ROS one, but: human.body_key_points_with_prob is of type list, not of type openpose_ros_msgs/PointWithProb. So the list contains openpose_ros_msgs/PointWithProb instances.

To access those, you'll need to use the [] operator to index into the list. Something like:

x = human.body_key_points_with_prob[0].x

would give you the .x attribute of the first entry in the list.

You would obviously first have to make sure that the list actually contains any items.

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Originally posted by gvdhoorn with karma: 86574 on 2018-04-04

This answer was ACCEPTED on the original site

Post score: 3

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Original comments

Comment by tolga-uni-lu on 2018-04-04:
Thank you for your answer.